In inductive power transfer (IPT) systems, power is transferred wirelessly by mutual induction between a primary conductor (variously known as a conductive path, pad, or track depending upon its form and/or application) supplied by an alternating-current (AC) power supply (the power supply and track together forming a primary side of the IPT system), and one or more pick-up circuits (forming the secondary side of the system) inductively coupled with the track and electrically coupled to a load to supply power thereto.
While an IPT system bears some superficial similarities with a transformer, there are in fact several key differences. In particular, a transformer is used to step an AC voltage up or down and/or galvanically isolate two circuits, whereas the purpose of an IPT system is used to transfer power wirelessly or contactlessly. Unlike a tightly-coupled conventional iron-core transformer with a coupling coefficient of (or near) unity (i.e. k=1), an IPT system is generally loosely-coupled and therefore has a lower coupling coefficient (e.g. commonly k<0.5) which may also vary dynamically during use if the pick-up circuit is not physically constrained to maintain a fixed alignment with the primary conductor. To optimise efficiency of the contactless or wireless power transfer, the primary power supply and secondary pick-up circuits of an IPT system are both tuned to the same resonant frequency. The resonant frequency is commonly somewhere between 10 and 40 kHz, for example.
The pick-up circuit of the IPT system therefore generally comprises a tuned or resonant circuit comprising at least a pick-up coil and a tuning capacitor. Two typical pick-up topologies are the series-tuned pick-up, in which the tuning capacitor is provided in series with the pick-up coil, or more commonly the parallel-tuned pick-up, in which the tuning capacitor is provided in parallel with the pick-up coil.
When the primary conductor is energised with an AC current at an appropriate frequency, a voltage is induced in the pick-up coil which is inductively coupled therewith. For optimum efficiency, the resonant circuit is thus tuned to resonate at the frequency of the AC current.
In many cases, the pick-up is required to supply a DC current to a load and the pick-up circuit will often be provided with a rectifier to rectify the AC current in the resonant circuit.
An example of a simple parallel-tuned IPT pick-up circuit of the prior art is shown in FIG. 1. The pick-up circuit is inductively coupled with primary track conductor/inductor LT, which induces a voltage in pick-up coil L1. Tuning capacitor C1 is provided in parallel with the pick-up coil, forming a resonant circuit therewith. A bridge rectifier, comprising diodes D1-D4, rectifies the AC current in the resonant circuit, and supplies a DC voltage to the output via DC inductor or choke LDC which acts to smooth the rectified output current. In the illustrated example, the pick-up circuit is coupled to a load, represented by VLoad. The load may comprise a battery, for example.
One potential disadvantage with the parallel-tuned pick-up circuits of the prior art, such as that shown in FIG. 1, is that they have a constant output current for a given AC track current and inductive coupling. In the parallel-tuned IPT pick-up topologies of the prior art, the output current of the pick-up is fundamentally limited by the short circuit current of the pick-up coil at a given coupling. This limits the output power at a given voltage and makes it difficult and inefficient to power low voltage, high current loads. It is also difficult to design an efficient pick-up which can handle a wide range of output voltages at a constant output power. If more current is required, then the track current must be increased or inductance of the pick-up coil reduced by reducing the number of turns, for example, of the resonant circuit in the pick-up.